Synthesis, biophysical characterization and anti-HIV activity of glyco-conjugated G-quadruplex forming oligonucleotides

Novel hybrid oligonucleotides carrying the G-quadruplex-forming d(5′TGGGAG3′) sequence, conjugated with mono- or disaccharides at the 3′ or 5′-end through phosphodiester bonds, have been synthesized as potential anti-HIV agents, via a fully automated, online phosphoramidite-based solid-phase strategy. CD-monitored thermal denaturation studies on the resulting quadruplexes indicated the insertion of a single monosaccharide at the 3′- end as the optimal modification, conferring improved stability to the quadruplex complex. In addition, the 3′- conjugation with glucose or mannose converted the anti-HIV inactive unmodified oligomer into active compounds. On the contrary, the 5′-tethering with these monosaccharides, as well as the conjugation, either at the 5′ or 3′- end, with sucrose, were in all cases detrimental to quadruplex stability and did not improve the biological activity. On the basis of the assumption that the kinetically and thermodynamically favored formation of the quadruplex complex is a prerequisite for efficient antiviral activity, a novel bis-conjugated oligonucleotide was designed. This combined a mannose residue at the 3′-phosphate end with bulky aromatic tert-butyldiphenylsilyl (TBDPS) group at the 5′-end, previously shown to markedly favor the formation of quadruplex complexes. The 5′,3′-bisconjugated 6-mer, for which a detailed biophysical characterization has been carried out, resulted in 3-fold greater antiviral activity against HIV-1 than the sole 3′-glyco-conjugated oligonucleotide.